Nitrate and Ammonium Nutrition Modulates the Photosynthetic Performance and Antioxidant Defense in Salt-Stressed Grass Species

Salt stress severely impairs plant growth and productivity. Here, the salinity response modulation by nitrogen nutrition forms was evaluated in two important grass C4 species (sorghum and maize). The crops were grown under nitrate (NO3−) or ammonium (NH4+) nutrition and subjected to salinity with 75 mM NaCl for 7 days. Growth, gas exchange, chlorophyll content, chlorophyll fluorescence, oxidative damage, and antioxidant enzyme activities were evaluated. In salt absence, maize plants displayed low shoot dry mass under NH4+ nutrition compared to NO3− supply; this injury was intensified by salt exposure. NH4+-grown stressed plants’ elevated sensibility is associated with decreased photosynthetic performance due to photochemical limitations. Although the NH4+-grown stressed maize plants activated superoxide dismutase (SOD) and catalase (CAT) antioxidant enzymes, the mechanisms were inefficient to prevent the oxidative damage, resulting in increased malondialdehyde (MDA) and electrolyte leakage (EL) in the leaves. In sorghum, dry mass was also decreased by salinity, except for NO3−-grown plants, at least in the evaluated time point. Salt-stressed sorghum plants showed unaltered CO2 assimilation under NH4+ nutrition, a response closely related to maintenance of photosystem II efficiency. Additionally, NH4+-stimulated SOD, guaiacol peroxidase (G-POD), and CAT enzymes’ integrated activities were critical in scavenging reactive oxygen species (ROS) under salinity once the increase in MDA and EL was less prominent than reported for maize plants. Findings clearly evidence that grass species display contrasting responses to salinity and the different N forms, highlighting the high susceptibility of salt-stressed maize plants under NH4+ nutrition and an opposite response for sorghum.